An Fe-36% Ni system alloy of Invar, 42-nickel alloy of Fe-42% Ni system alloy, Koval alloy of Fe-29% Ni-17% Co system alloy, and other types of alloys have hitherto been known as alloys for use as components which require low coefficients of thermal expansion. Though they have low coefficients, these alloys have a low degree of strength at room and elevated temperatures; consequently, they cannot be used as components which require high strength at room and elevated temperatures.
A type of alloy corresponding to Incoloy 903 is disclosed in JP-B-41-2767, and alloys improved over Incoloy 903 are disclosed in JP-A-50-30729, 50-30730, 59-56563, 60-128243, U.S. Pat. No. 4,200,459, etc. These types of alloys are known for their high-temperature strength enhanced by adding precipitation strengthening elements, such as Al, Ti and Nb, and also for their low coefficients of thermal expansion, smaller than those of ordinary austenitic alloys, but much greater than the coefficients of thermal expansion of the initially mentioned alloys.
Alloys disclosed in JP-A-61-23118, 2-70040, etc. have strengths and coefficients of thermal expansion which ar intermediate to those of Incoloy 903-system alloys and Koval-system alloys.
With an increase in operating temperature for gas turbine components, there has been a trend in recent years toward an increasing demand for higher strength materials capable of maintaining a constant clearance between components or members from room to elevated temperatures, and for improvement in the properties of joining metallic materials to materials, having low coefficients of thermal expansion, such as ceramics and cemented carbides
Such alloys are used as, for example, collars for joining the rotor shaft of automobile turbochargers with the ceramic blades. The alloys are also used as components of gas turbines, such as compressor housings, exhaust ducts and sealing media; as sleeves for die casting aluminum each composed of a ceramic inner cylinder and an outer cylinder made of a superalloy with a low coefficient of thermal expansion; and as edge tools, as cushioning materials of cemented carbides and alloys, made of cemented carbides utilizing alloys with low thermal expansion.
Incoloy 903 disclosed in JP-B-41-2767 has been put into practical use for such needs. However, it has a high notch-sensitivity at operating temperatures of about 500.degree. C., and there is a marked difference between the notch and the smooth creep rupture strengths at 500.degree. C., thus causing a problem.
The alloys disclosed in JP-A-50-30729, 50-30730, 59-56563, 60-128243, U.S. Pat. No. 4,200,459, etc. mentioned previously are proposed as improved alloys to solve the problem. Of the improved alloys, only Incoloy 909 has been put into practical use. However, although it is superior to Incoloy 903 in notch rupture strength, it has a coefficient of thermal expansion substantially equal to that of Incoloy 903; the thermal expansion is not desirably decreased.
On the other hand, although alloys disclosed in JP-A-61-23118 and 2070040 display coefficients of thermal expansion lower than that of Incoloy 909, these alloys have high-temperature strengths lower than that of Incoloy 909.
In view of the above problems, the object of the present invention is to provide superalloys with low coefficients of thermal expansion which are capable of satisfying the need for the greatest high-temperature strength and the lowest coefficient of thermal expansion of all the conventional superalloys with low thermal expansion.